Apparatus for manufacturing metal tube covered optical fiber cable and method therefor

ABSTRACT

An apparatus for manufacturing a metal tube covered optical fiber cable, including an assembly (2) having a plurality of roller pairs, for causing both side edges of a metal strip (1) to abut against each other to form the metal strip into a metal tube, a laser welding means (7) for radiating a laser beam to abutment portions of the metal tube to bond the abutment portions to obtain a sealed metal tube (1c), and an optical fiber guiding means (6) for guiding an optical fiber (5) in a formed metal tube (1a) comprises 
     an extra length control means comprising 
     a tension adjusting means (15) for the metal strip (1) arranged in the upstream of the assembly (2) and a tension adjusting means (14) for the optical fiber (5) arranged in the upstream of the optical fiber guide means (6), and 
     a traction means (11, 13) including tension variable means (11) for the metal tube covered optical fiber cable (1d).

TECHNICAL FIELD

The present invention relates to an apparatus for manufacturing a metaltube covered optical fiber cable and a method therefor.

BACKGROUND ART

An optical fiber and a bundle of optical fibers are variously modifiedin accordance with the conditions under which they are used. However, atension member must be used with an optical fiber to assure a highstrength in some cases. When water permeates an optical fiber cable, itsstrength may be degraded. When an optical fiber cable is to be installedin the bottom of a sea or the bottom of the water, in order to assure asufficient installation tension and a high water resistance, an opticalfiber cable must be used in a jacket structure in which an optical fibercable is covered with a thin metal tube.

In this manner, there is provided an apparatus and method ofcontinuously manufacturing a metal tube covered optical fiber cable, asdisclosed in Published Unexamined Japanese Patent Application No.64-35514.

This apparatus for manufacturing the metal tube armored optical fibercable forms a continuously fed flat metal strip into a metal tube havinga longitudinal gap at a top portion. A guide tube is inserted into themetal tube through this gap of the metal tube, and an optical fiber isguided into the metal tube through the guide tube. After the gap of themetal tube having received this optical fiber is closed, the metal tubeis supplied to a laser welding unit.

The laser welding unit causes a guide roller to align the abutment edgeportions of the top portion of the metal tube to each other. A laserbeam having a focal point at a position outside the range of theabutment portions is radiated to weld the abutment portions. Since thelaser beam is focused outside the rang of the abutment portions, theabutment portions can be welded without protecting the optical fiberwith a heat-shielding member.

This metal tube containing the optical fiber cable is drawn to have apredetermined outer diameter, and the drawn tube is continuously woundaround a capstan.

During drawing of this metal tube, an inert gas is supplied to the guidetube to carry the optical fiber cable by the viscosity resistance of thegas. While the metal tube is kept engaged with the capstan, the opticalfiber cable is blown outward against the inner surface of the metaltube, so that the length of the optical fiber cable is set larger thanthat of the metal tube. The optical fiber cable is not kept taut toprevent the optical fiber cable from stress caused by an installationtension or the like.

In order to protect the optical fiber cable from water entering from ahole formed in a damaged metal tube, a gel is injected inside the metaltube. More specifically, after the optical fiber cable is blown outwardagainst the inner surface of the metal tube by the inert gas at thecapstan, the gel is injected from a gel guide tube different from theguide tube for guiding the optical fiber cable.

Optical fiber cables are used in a variety of application conditions andat various temperatures. The thermal expansion coefficient of the metaltube is much larger than that of the optical fiber cable. For thisreason, when optical fiber cables are used at high temperatures, atension acts on the optical fiber cable due to a difference inelongations of the metal tube and the optical fiber cable to damage theoptical fiber cable. This also occurs when a cable is installed at ahigh tension, e.g., in installment at the bottom of a sea.

To the contrary, when optical fiber cables are used at low temperatures,the optical fiber cable is brought into contact with the inner wallsurface of the metal tube having a large shrinkage amount due to a largedifference between the degrees of shrinkage of the metal tube and theoptical fiber cable. The optical fiber cable directly receives a sidepressure from the inner wall of the metal tube. Irregular bending forceshaving short periods act on the optical fiber cable to cause a so-calledmicrobend loss, thereby attenuating a signal transmitted through theoptical fiber cable.

In order to prevent damage and the like, the optical fiber cable isblown outward against the inner wall surface of the metal tube while themetal tube is kept engaged with the capstan, so that the length of theoptical fiber cable is set larger than that of the metal tube after thecable is straightened for use.

In this case, however, a difference between the length of the opticalfiber cable and the length of the metal tube (to be referred to as anextra length hereinafter) is determined by the outer diameter of thecapstan and a difference between the inner diameter of the metal tubeand the outer diameter of the optical fiber cable. The extra lengthcannot be arbitrarily controlled, and the optical fiber cable may bedamaged depending on application conditions.

As described above, while the metal tube is kept engaged with thecapstan, the optical fiber cable is blown outward against the innersurface of the metal tube by an inert gas to provide an extra length tothe optical fiber cable. For this reason, when a gel is to be injectedinto the metal pipe it must be injected while the optical fiber cable iskept blown outward against the inner wall of the metal tube due to thefollowing reason. That is, even if the gel is injected and then theinert gas is supplied, the gel causes resistance to fail to give theextra length to the optical fiber cable. When a gel is to be injected, agel guide tube is required in addition to the optical fiber cable andthe inert gas guide tube. Since these two guide tubes must besimultaneously inserted into the metal tube, the inner diameter of themetal tube is increased. In order to obtain a thin tube from this tube,the drawing amount is increased. Metal tubes may not be occasionallythinned in accordance with diameters of optical fiber cables, resultingin inconvenience.

[Disclosure of Invention]

The present invention has been made to solve the above drawbacks, andhas as its object to provide an apparatus for manufacturing a metal tubecovered optical fiber cable and a method therefor, capable of preventingdamage to an optical fiber cable during welding abutment portions of ametal tube, capable of continuing the manufacturing operation for a longperiod of time, and capable of arbitrarily obtaining an extra length.

There is provided an apparatus for manufacturing a metal tube coveredoptical fiber cable, comprising an assembly, having a plurality ofroller pairs, for causing both side edges of a metal strip to aboutagainst each other to form the metal strip into a metal tube, laserwelding means for radiating a laser beam to abutment portions of themetal tube to bond the abutment portions to obtain a sealed metal tube,and optical fiber guiding means for guiding an optical fiber or anoptical fiber bundle into the formed metal tube, characterized bycomprising extra length control means comprising,

tension adjusting means, arranged in the upstream of the assembly, forvariably changing a tension of the metal strip in the upstream of theassembly and adjusting a tension of the metal tube,

tension adjusting means, arranged at an optical fiber guide inlet portof the optical fiber guide means, for variably adjusting a tension ofthe optical fiber cable, and

traction means having tension variable means for reducing a tension ofthe metal tube covered optical fiber cable and supplying the metal tubecovered optical fiber cable.

The traction means preferably continuously draws the metal strip, theformed metal tube, and the sealed metal tube incorporating the opticalfiber or optical fiber bundle through the assembly, the optical fiberguide means, the laser welding means, and drawing means, therebyreducing the tension of the metal tube covered optical fiber cable.

The tension variable means preferably comprises a capstan around whichthe metal tube covered optical fiber cable is wound a plurality oftimes.

In addition, the tension of the metal tube covered optical fiber cableat an outlet of the tension variable means is preferably adjusted by thetension adjusting means arranged in the downstream of the tensionvariable means.

Furthermore, the tension of the metal tube covered optical fiber cableat an inlet side of the tension variable means is preferably adjusted bytension means arranged in the upstream of the tension variable means.

Extra length control is performed such that the tension of the opticalfiber cable is adjusted to a predetermined value and the tension of themetal strip is set variable, or the tension of the metal strip isadjusted to a predetermined value and the tension of the optical fibercable is set variable, thereby obtaining an arbitrary extra length.

According to the present invention, there is provided a method ofmanufacturing a metal tube covered optical fiber cable, comprising theforming step of forming a metal strip subjected to traction into a metaltube through forming rollers, the laser welding step of welding abutmentportions of the formed metal tube with a laser beam and forming theformed metal tube into a sealed metal tube, and the optical fiber guidestep of guiding an optical fiber or optical fiber bundle in the sealedmetal tube, comprising,

setting a tension of the metal strip before assembly step variable toadjust a tension of the metal tube after a drawing step, and setting atension of the optical fiber cable in the optical fiber guide stepvariable and adjusting the tension of the optical fiber cable guidedinto the metal tube,

in the traction step, reducing tensions of the metal tube and theoptical fiber cable to control an extra length of the metal tube coveroptical fiber cable.

In addition, it is preferable to reduce the tension of the metal tubeoptical fiber cable while the metal tube optical fiber is kept subjectedto traction with a capstan around which the metal tube covered opticalfiber cable is wound a plurality of times.

In addition, the step of reducing the tension of the metal tube coveredoptical fiber may be preferably performed after a tension is applied tothe metal tube.

According to the present invention, when the metal tube covered opticalfiber cable is to be manufactured, the tension of the metal tube coveredoptical fiber at the inlet side of the tension variable means isadjusted by the tension of the metal strip formed into the metal tubeand the tension of the optical fiber cable guide into the metal tube toprovide a difference between the tensions of the sealed metal tube andthe optical fiber cable therein. The difference in tension is reduced bythe tension variable means to obtain a difference between an elongationof the sealed metal tube at the inlet side of the tension variable meansand an elongation of the optical fiber cable in the metal tube.Therefore, the length of the optical fiber cable relative to the metaltube is arbitrarily adjusted by the difference in elongation.

In addition, since the capstan around which the metal tube coveredoptical fiber cable is wound a plurality of times is used as the tensionvariable means, the tension can be easily reduced while the metal tubecovered optical fiber cable is kept subjected to traction.

Furthermore, the tension of the metal tube at the inlet or outlet sideof the tension variable means is set variable to increase an extralength control range.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view of an overall arrangement showing an embodiment of thepresent invention,

FIGS. 2A and 2B are sectional views showing a metal tube in differentforming steps,

FIGS. 3A, 3B, and 3C are side views showing forming roller pairs of asecond assembly,

FIG. 4 is a view showing an arrangement of an optical fiber guide means,

FIG. 5 is a view showing an arrangement of a laser welding means,

FIG. 6 is a side view showing a guide shoe,

FIGS. 7A and 7B show a tension variable means and a tension adjustingmeans, respectively, in which FIG. 7A is a plan view thereof, and FIG.7B is a front view thereof,

FIG. 8 is a view showing abutment portions of a metal tube,

FIG. 9 is a graph showing a relationship between a tube outer diameterand a rear bead width,

FIG. 10 is a graph showing a relationship between a tube wall thicknessand a rear bead width,

FIGS. 11, 12, and 13 are graphs showing relationships between focalpoint shift amounts and welding rates, respectively,

FIGS. 14, 15, and 16 and FIG. 17 are views for explaining extra lengthcontrol operations,

FIG. 18 is a view showing a part of another layout of the optical fiberguide means,

FIG. 19 is a view showing part of another embodiment,

FIG. 20 is a view for explaining a leaf spring mechanism,

FIG. 21 is a view for explaining a state in which the metal tube is keptat an upper position,

FIG. 22A is a view for explaining a bent state of a guide tube,

FIG. 22B is a view for explaining a guide tube positioning mechanism,and

FIG. 23 is a view for explaining a positioning state of the metal tubeat a welding position.

BEST MODE OF CARRYING OUT THE INVENTION

FIG. 1 is a view of an overall arrangement showing an embodiment of thepresent invention. As shown in FIG. 1, an apparatus for manufacturing ametal tube covered optical fiber cable comprises an assembly 2constituted by first and second assemblies 3 and 4 for forming a metalstrip 1 and forming the metal strip into a metal tube so as to abut bothside edges of the strip, an optical fiber guide means 6, arrangedbetween the first and second assemblies 3 and 4, for guiding an opticalfiber cable 5 into the formed metal tube, and a laser welding means 7arranged as the next stage of the assembly 2.

A measuring unit 8 and a drawing means 9 are arranged next to the laserwelding means 7. A traction means comprising a tension variable means 11and a tension adjusting means 13 for the metal tube covered opticalfiber cable 12 is arranged between the drawing means 9 and a cablewinding machine 10.

The tension variable means 11, the tension adjusting means 13, and atension adjusting means 14 for the metal strip 1 and a tension adjustingmeans 15 for the optical fiber cables, which latter two are arranged inthe upstream of the assembly 2, constitute an extra length control meansfor controlling a so-called extra length, i.e., the length of theoptical fiber cable relative to the metal tube.

The first assembly 3 constituting the assembly 2 comprise a plurality(e.g., five) of roller pairs 31a to 31e continuously aligned with eachother. The forming roller pairs 31a to 31e sequentially have differentforming surfaces and form the continuously fed metal strip 1 into asubstantially U-shaped metal tube 1a having a longitudinal gap at itstop portion, as shown in a sectional view of FIG. 2A.

Similarly, the second assembly 4 comprises a plurality (e.g., five) offorming roller pairs 41a to 41e continuously aligned with each other. Asshown in FIGS. 3A, 3B, and 3C, fins 17 gradually reduced in size areformed in the upper rollers corresponding to the forming roller pairs41a to 41d of the previous stages. A gap 16 of the metal tube la isengaged with each fin 17 so that the gap 16 is located at the top pointof the metal tube 1a and the gap 16 is gradually reduced by the fins 17.Abutment portions 18 of the metal tube 1a are brought into contact witheach other by the forming roller 41e of the last stage, thereby forminga metal tube 1b almost tightly closed at the abutment portions 18.

As shown in a partial sectional view of FIG. 4, the optical fiber guidemeans 6 comprises a guide tube 61 inserted into the metal tube 1b toguide the optical fiber cable 5, and an inert gas supply tube 63connected to the guide tube 61 through a tube connector 62 and an inertgas supply tube connector 62.

The guide tube 61 is made of a metal excellent in thermal conductivity,such as copper or a copper alloy. The outer diameter of the guide tube61 is smaller than the inner diameter of the metal tube 1b. The guidetube 61 is inserted from the gap 16 of the metal tube 1 between thefirst and second assemblies 3 and 4. A distal end of the guide tube 61passes through the laser welding means 7 and is located in front of aneddy current probe 81 of the measuring unit 8. The distal end of theguide tube 61 is inserted in front of the eddy current probe 81 asdescribed above because probe precision is adversely affected when theguide tube 61 reaches the eddy current probe 81.

When a probe measurement result is not adversely affected by insertionof the guide tube 61 passing through the eddy current probe 81, forexample, when the diameter of the metal tube is large and the guide tube61 is in contact with the inner wall surface of the metal tube at aposition opposite to a probe position, the guide tube 61 may be insertedand reaches a position passing through the position of the eddy currentprobe 81, that is, it may be inserted in front of the drawing means 9.

The guide tube 61 can be provided with a leaf spring mechanism 611 (FIG.19) facing upward in front of and/or behind the laser beam radiationposition of the laser welding means 7 and elastically contacting theinner wall surface of the metal tube 1b, or as shown in FIG. 20, themetal tube 1b can be located at a position higher by a predetermineddistance in front of and/or behind the laser beam radiation position.Alternatively, a downward elastic force can be applied to the guide tube61 itself to bring the guide tube 61 into contact with the inner wall ofthe metal tube 1b at a position opposite to the laser beam radiationposition.

Elastic contact between the guide tube 61 and the inner wall of themetal tube 1b can be easily achieved, as shown in FIG. 22. The guidetube 61 is curved from a state I to a state II by elasticity of theguide tube 61 itself against the nature of straight extension of theguide tube 61 (FIG. 22(A)), the guide tube 61 is brought into contactwith the inner wall of the metal tube 1b in the state II (FIG. 22(B)),and the optical fiber guide means 6 is fixed at an appropriatelyposition to maintain a curved state. At this time, a positioningmechanism 612 constituted by a spring mechanism or the like ispreferably added to the optical fiber guide means 6, as needed.

When the metal tube 1b is to be located at a position higher by thepredetermined distance in front of the laser beam radiation position, apositioning unit 71 (to be described later) is finely adjusted. On theother hand, when the guide tube 61 is located at a position higher bythe predetermined distance behind the laser beam radiation position, asupport roll stand 82 (to be described later) is finely adjusted.

As shown in the arrangement of FIG. 5, the laser welding means 7comprises a positioning unit 71 for positioning the metal tube 1b and alaser welding unit 72.

The positioning unit 71 comprises, e.g., two sets of guide shoes 73 and74, a CCD seam monitor 75 arranged between the guide shoes 73 and 74,and micrometers 76 for finely adjusting vertical and horizontalpositions of the guide shoes 73 and 74.

The guide shoe 73 (74) comprises an upper shoe 73a (74a) and a lowershoe 73b (74b), as shown in the side view of FIG. 6. The upper shoe 73a(74a) has a flat surface which is brought into contact with the metaltube 1b. The lower shoe 73b (74b) has, e.g., a V-shaped groove engagedwith the metal tube 1b and is biased upward by a spring.

The laser welding unit 72 comprises a laser radiation means 77, and agas seal means 78 for sealing a welding position of the metal tube 1bwith an inert gas such as argon gas.

The laser radiation means 77 is connected to, for example, a carbondioxide laser device, guides and focuses a laser beam through an opticalsystem, and emits the focused laser beam on the surface of the metaltube 1b at an angle of about 90°. A focal point of the laser beamradiated on the surface of the metal tube 1b is adjusted to be locatedat a position below the abutment portions 18 of the metal tube 1b, i.e.,a position inside the metal tube 1b.

The measuring unit 8 arranged next to the laser welding means 7comprises a support roll stand 82, a speedometer 83, and the eddycurrent probe 81 and checks a welded state or the like.

The drawing means 9 comprises a roller die and draws a welded and sealedmetal tube 1c to have a predetermined outer diameter, thereby obtaininga thin metal tube 1d corresponding to the outer diameter of the opticalfiber cable 5.

The tension variable means 11 arranged in the upstream of the drawingmeans 9 comprises a capstan having, e.g., a pair of rolls 11a and 11b,as shown in FIGS. 7A and 7B. The surface of one roll 11a is formedsmooth, and the surface of the other roll 11b has a plurality ofgrooves. The metal tube 1d is wound around the capstan withoutoverlapping the turns of the metal tube 1d. Similarly, the tensionadjusting means 13 comprises a dancer roll stand having a pair of rolls13a and 13b. One roll 13b is moved in a direction indicated by an arrowto change a distance between the rolls 13a and 13b so that the tensionis adjusted, thereby adjusting a tension of the metal tube coveredoptical fiber cable 12 in the downstream of the capstan 11.

The tension adjusting means 14 and 15 for adjusting the tension of themetal strip 1 fed to the assembly 2 and the tension of the optical fibercable 5 supplied to the optical fiber guide inlet of the guide tube 61comprise dancer stands, respectively. The tensions of the dancer stands14 and 15 are variably adjusted by moving weights linked to pulleys 14aand 15a engaged with the metal strip 1 and the optical fiber cable 5.

An operation of manufacturing the metal tube covered optical fiber cable12 by the manufacturing apparatus having the above arrangement will bedescribed in an order of manufacturing steps.

(1) Forming Step

The metal strip 1 is continuously supplied while the metal strip 1 isadjusted by the dancer stand 14 to have a predetermined tension. Thefirst assembly 3 of the assembly 2 forms the supplied metal strip 1 intothe metal tube 1a having the longitudinal gap 16 at the top portion. Themetal tube 1a is supplied to the second assembly 4, and the gap 16 issequentially engaged with the fins 17 of the forming roller pairs 41a,41 and is gradually reduced. The gap 16 is eliminated by the formingroller pair 41e of the last stage, so that the abutment portions 18 areperfectly closed, thereby obtaining the metal tube 1b. When the metaltube 1b passes through the last forming roller 41e, a small gap 18a (tobe described later) is actually formed between the abutment portions 18.The gap 18a was not changed in a path from the forming roller 41e to thelaser beam radiation position, as detected by an another CCD monitor(not shown).

(2) Optical Fiber Cable Insertion Step

Meanwhile, the optical fiber cable 5 adjusted by the dancer stand 15 tohave a predetermined tension is continuously supplied through the guidetube 61 which has been inserted from the gap 16 of the metal tube 1abetween the first and second assemblies 3 and 4. At the same time, argongas flows into the guide tube 61 from the inert gas supply tube 63connected to the guide tube 61.

(3) Laser Welding Step

The metal tube 1b inserted into the guide tube 61 is supplied to thelaser welding means 7. Since the metal tube 1b supplied to the laserwelding means 7 is positioned by the fins 17 of the forming roller pairs41a and 41b, the abutment portions 18 can be perfectly aligned with theposition of the laser beam emitted from the laser beam radiation means77.

The metal tube 1b supplied to the positioning unit 71 of the laserwelding mean 7 is engaged with and guided along the grooves of the guideshoes 73 and 74. Lateral shifts, rotation and zig-zag movement of themetal tube 1b can be prevented. The positional deviations of theabutment portions 18 were observed on the CCD monitor 75. When a guideroller was used, the abutment portions 18 were moved within the range of±100 μm due to torsion. However, when the guide shoes were used, theabutment portions were move by only ±15 μm.

Subsequently, the CCD seam monitor 75 continuously detects the positionsof the abutment portions 18 of the metal tube 1b, and the micrometer 76is automatically or manually operated in accordance with a detectionresult to move the guide shoes 73 and 74, thereby finely adjusting thatthe abutment portions 18 are located at a predetermined position withrespect to the focal point of the laser beam.

The role of the positioning unit 71 will be described below. Aspreviously described, the guide shoes 73 and 74 in the positioning unit71 prevent rotation and zig-zag movement of the metal tube lb and guide,to the laser beam radiation position, the abutment portions 18accurately positioned by the rollers 41a to 41d with fins with respectto the laser radiation position without causing zig-zag movement of themetal tube 1b. As previously described, it is possible to distance infront of the laser beam radiation position upon adjustment of thepositioning unit 71. As a result, elastically tight contact between theguide tube 61 and the inner wall surface Of the metal tube 1b can beperformed to minimize an adverse influence of laser welding (to bedescribed later) and allow a continuous manufacturing operation for along period of time.

As shown in FIG. 23, the positioning unit 71, i.e., the metal tube 1b ismoved upward or downward by a predetermined distance or more (within thelimit of elasticity) with respect to a path line by using the supportrolls 82a and 82b of the support roll stand 82 and the final formingroller pair 41e as two support points, so that the metal tube 1bconstitute two sides of a substantial triangle.

At this time, a light tension acts on the metal tube 1b located betweenthe support roll stand 82 and the final forming roller pain 41e. Thisindicates that the positioning unit 71 also serves as a means foradjusting a tension of the metal tube (particularly 1c and 1d) as in thetension adjusting means 14 for the metal strip (to be described indetail later). Vibrations of the metal tube 1b at the laser weldingposition (marks X in FIG. 23) can be suppressed.

In practice, another CCD monitor (not shown) was located at a positioninclined from the laser radiation position by 90° with respect to theCCD seam monitor 75 and the path line as the center, and verticalvibrations of the metal tube 1b were observed. As a result, when theguide shoes 73 and 74 in the positioning unit 71 are open, the metaltube 1b was vibrated in the range of about ±100 to about ±150 μm.However, when the metal tube 1b was fixed by the guide shoes 73 and 74,the metal tube 1b was vibrated within the range of about ±20 to about±30 μm. When the positioning unit 71 is adjusted as indicated by a state(A) or (B) in FIG. 23, the metal tube 1b was confirmed to be vibratedwithin the range of about ±5 μm.

When the elastic contact between the guide tube 61 and the inner wallsurface of the metal tube 1b is taken into consideration, thepositioning unit 71 is more preferably adjusted in the state (A) thanthe state (B).

With the above adjustment operations, highly precise welding control canbe performed, adverse welding influences can be minimized, a long-termoperation is allowed.

The metal tube 1b having abutment portions 18 whose positions areadjusted is supplied to the laser welding unit 72. The laser weldingunit 72 radiates a laser beam from the laser radiation means 77 to weldthe abutment portions 18 while supplying argon gas from the gas sealmeans 78 to the abutment portions 18 of the metal tube 1b. The innersurface of this welded portion is sealed by argon gas flowing within theguide tube 61 and reversely flowing from the distal end of the guidetube 61.

Before and after the laser beam radiation position, the guide tube 61which guides the optical fiber cable 5 is located in elastic contactwith the inner wall of the metal tube 1b at a position opposite to thelaser beam radiation position, and a gap is formed between the innersurfaces of the abutment portions 18 and the guide tube 61. The opticalfiber cable 5 is shielded from heat by means of this gap and the guidetube 61, thereby minimizing the thermal influence on the optical fibercable 5.

When the guide tube 61 is to be located at a position opposite to theabutment portions of the metal tube at the laser welding portion, thepositioning unit 71 can be adjusted to locate the metal tube 1b higherthan the path line, and the layout of the guide tube 61 can be moreflexible.

In addition, the optical fiber cable 5 is cooled by the argon gasflowing in the guide tube 61 and the argon gas reversely flowing fromthe guide tube 61, thereby minimizing a temperature rise of the opticalfiber cable 5.

For example, when the guide tube 61 was in contact with the abutmentportions 18 at the laser radiation position, the temperature near theoptical fiber cable 5 heated to a temperature of 600° C. or more canbecome about 115° C. to about 135° C. due to the presence of the gap.When argon gas flowed in the guide tube 61, the temperature was furtherreduced to about 100° C.

When the above gap is formed, an adverse influence on welding, which iscaused by sputter components deposited on the guide tube 61, can belagged. Therefore, welding can be stably performed for a long period oftime.

Since the laser beam emitted from the laser radiation means 77 isadjusted so that the focal position of the laser beam is set inside themetal tube 1b, an excessive increase in power density of the laser beamincident on the abutment portions 18 can be prevented, and stablewelding can be performed. When the focal position is focused inside themetal tube 1b, and once a cavity is formed, a laser beam reflected by acavity wall is focused toward the bottom of the cavity, so that a deepcavity is formed. A welding width can be set to be almost constant, anda rear bead width can be made small.

Since the focal point shift amount (defocusing amount) of the laser beamradiated with a predetermined power is controlled by setting theradiation power density to fall within the predetermined range, and thefocal point shift amount, i.e., the welding rate is determined inaccordance with the radiation power density, the rear bead width can bereduced to suppress the influences of sputter components.

A minimum value b_(min) of the rear bead width is determined by acondition that nonwelded portions are not left in the abutment portions18. A maximum value b_(max) of the rear bead width is determined by thelimit vibration of about ±5 (μm) was found to occur between the laserbeam and the small gap 18a due to small vibrations of the apparatus.

The minimum width b_(min) of the rear bead becomes 10d±5 (μm). Forexample, when the outer diameter of the metal tube 11b is 1 (mm), theminimum width b_(min) of the rear bead becomes 20 (μm).

The minimum width b_(min) =10d±5 of the rear bead is exemplified withuse of the metal tube 1b consisting of Fe-group stainless having thelongitudinal modulus of elasticity of 18,000 (kg/mm²). However, whenFe-group stainless or an Ni-group alloy having a longitudinal modulus ofelasticity of more than 18,000 (kg/mm²) is used, the rear bead width isset to be larger than the minimum width b_(min), and good welding freefrom the nonwelded portions can be performed.

The limit free from the sputter components even in a long-term operationis determined by a shape of a welding portion. A relationship between atube wall thickness t (mm) and the rear bead width b (μm) is shown inFIG. 10 when a laser beam having a power of 400 (w) is emitted in thesmall gap 18a to perform welding. The tube wall thickness t is plottedalong the abscissa of FIG. 10, and the rear bead width b is plottedalong the ordinate. Referring to FIG. 10, a circle indicates a statewherein no sputter influence is found and welding can be continuouslyperformed for a long period of time, e.g., 10 hours. A cross indicates astate wherein the sputter influence occurs and welding cannot beperformed for a long period of time. The long period of time, i.e., 10hours corresponds to a maintenance timing in an actual operation. Thistime does not indicate the limit time free from the sputter influence.

A straight line B indicates the limit free from the sputter influenceeven in an operation for a long period of time and is represented byb=1000(t/2). When the tube wall thickness t is 0.1 (mm) an allowablemaximum width b_(min) of the rear bead width can be 50 (μm).

As described above, when the laser beam having a power of 400 (W) isused, and the metal tube 1b has a wall thickness of 0.1 (mm) and anouter diameter of 1 (mm), the width b of rear bead is controlled to fallwithin the range of 20 to 50 (μm) to perform welding. The sputterinfluence can be suppressed even if welding is performed for a longperiod of time. Therefore, welding free from defects can be continuouslyperformed.

In order set the rear bead width b within the predetermined range inthis manner, focal point shifting (defocusing) of the laser beamradiated on the abutment portions 18 must be performed in accordancewith the size of the metal tube 1b to control the radiation powerdensity.

The welding rate is determined by a focused laser beam spot size, i.e.,a focal point shift amount and in which sputter influences do not occurin a long-term operation.

Although the metal tube 1b is held by the guide shoes 73 and 74 at theposition of the laser welding means 7, the small gap 18a is formedbetween the abutment portions 18 of the metal tube 1b by a spring backat the position of the laser welding portion 72, as shown in FIG. 8. Thespring back which forms this small gap is influenced by rigidity of themetal tube 1b, i.e., an outer diameter d of the formed metal tube 1b.For example, a relationship between the outer diameter d (mm) and therear bead width b (μm) is examined and shown in FIG. 9 when a laser beamhaving a power of 400 (W) is incident in the small gap 18a while themetal tube 1b consisting of Fe-group stainless having a longitudinalmodulus of elasticity of 18,000 (kg/cm²) is perfectly fixed. The tubeouter diameter d is plotted along the abscissa in FIG. 9, and the rearbead width b is plotted along the ordinate. Referring to FIG. 9, acircle indicates a portion where a nonwelded portion is not formed, anda cross indicates a portion where a nonwelded portion is formed.Therefore, a straight line A indicates the limit at which nonweldedportions are not formed. The straight line A is given as b=10d.

In an actual apparatus, according to the observation with the CCD seammonitor 75, a relative an overlap ratio.

The size of the metal tube 1b is variously changed to conduct testsunder a condition that the minimum value satisfies b_(min) ≧10d±5 (μm)and the maximum value of the rear bead width satisfies b_(max)≧1000(t/2) and a condition that the minimum and maximum value do notsatisfy b_(min) ≧10d±5 (μm) and b_(max) ≧1000(t/2). The welding rate V(m/min) is plotted along the abscissa and the focal point shift amount F(mm) (absolute value) is plotted along the ordinate. Test results areshown in FIGS. 11, 12, and 13. FIG. 11 shows a case wherein the metaltube 1b has an outer diameter d of 3.5 (mm) and a tube wall thickness tof 0.2 (mm). FIG. 12 shows a case wherein the metal tube 1b has an outerdiameter d of 2.0 (mm) and a tube wall tube t of 0.15 (mm). FIG. 13shows a case wherein the metal tube 1b has an outer diameter d of 1.0(mm) and a tube wall thickness t of 0.1 (mm). Referring to FIGS. 11 to13, a circle indicates a case satisfying the above condition, and across indicates a case not satisfying the above condition. Their limitsare represented by curves A and B. The curve A represents the minimumvalue b_(min) =10d±5 (μm) of the optimal rear bead width. The curve Brepresents the maximum value b_(max) =1000(t/2) of the optimal rear beadwidth.

As shown in FIG. 11, when the metal tube 1b has the outer diameter d of3.5 (mm) and the tube wall thickness t of 0.2 (mm), the optimal range ofthe focal point shift amount F has a largest allowable value in therange of F=0.85 (mm) to F=1.45 (mm). By setting the focal point shiftamount within this range and the welding rate V to 4 (m/min), weldingcan be performed stably for a long period of time without any influenceof sputter components while the rear bead width b falls within thepredetermined range of 40 to 100 (μm).

Similarly, when the metal tube 1b has the outer diameter d of 2.0 (mm)and the tube wall thickness t of 0.15 (mm), the focal point shift amountF is set to fall within the range of 0.8 to 1.3 (mm). In this case,welding is performed while the welding rate V is set to 6 (m/min). Whenthe metal tube 1b has the outer diameter d of 1.0 (mm) and the tube wallthickness t of 0.1 (mm), the focal point shift amount F is set to fallwithin the range of 0.7 to 1.1 (mm). In this case, when the welding rateV is set to 10 (m/min), welding can be continuously and stablyperformed. The small gap 18a between the abutment portions 18 of themetal tube 1b is slightly changed in accordance with an extra lengthcontrol condition described separately and the method of setting thepositioning unit 71 (A and B in FIG. 23). For example, when the state Ain FIG. 23 is set, the size of the small gap 18a is increased. However,when the state B in FIG. 23 is set, the size tends to be reduced.

In practice, within the measurement mesh range of the presentapplication, however, an influence of a change in the small gap 18a wasfound to rarely influence the welding result.

(4) Measurement and Drawing Step

The sealed metal tube 1c having the welded abutment portions 18, asdescribed above, is supplied to the measurement unit 8. In themeasurement unit 8, the passing speed, i.e., the welding rate V, of themetal tube 1c is measured by the speedometer 83 while the metal tube 1cis supported by the support roll stand 82. The welded state is checkedby the eddy current probe 81.

The metal tube 1c passing through the eddy current probe 81 is drawn bythe drawing means 9 to have a diameter corresponding to the outerdiameter of the optical fiber cable 5 incorporated in the metal tube 1c,thereby obtaining the metal tube covered optical fiber cable 12. Duringdrawing of the metal tube 1c by the drawing means 9, since only oneguide tube 61 is inserted into the metal tube 1c just at the inlet sideof the eddy current probe 81 and since the guide tube 61 does not extendup to the drawing means 9, the metal tube 1c can be made thin, and thediameter of the metal tube 1c can be easily reduced.

(5) Traction and Winding Step

The metal tube covered optical fiber cable drawn by the drawing means 9is wound by the cable winding machine 10 through the tension variablemeans 11 and the tension adjusting means 13.

When the metal tube covered optical fiber cable 12 is to be wound, thesealed and diameter-reduced metal tube 1d must be engaged with theoptical fiber cable 5. For this purpose, prior to a continuousoperation, after the welded and sealed metal tube 1d is manually woundaround the capstans 11a and 11b of the tension variable means 11 by aplurality of times, it is subjected to traction. The distal end of themetal tube 1d is mounted on the cable winding machine 10 through thetension adjusting means 13. In this state, the distal end of the opticalfiber cable 5 is inserted just in front of the capstan 11a, and themetal tube 1d is pressed at this position, thereby engaging the opticalfiber cable 5 with the inner wall of the metal tube 1d. Thereafter, themetal tube 1d is wound while the capstans 11 are driven. The opticalfiber cable 5 together with the metal tube 1d is pulled from the guidetube 61, thereby winding the pulled product as the metal tube coveredoptical fiber cable 12. In the case where the optical fiber cable 5 andthe metal tube 1d can be wound together around the capstan 11a, it isnot necessary to engage the optical fiber cable 5 with the metal tube 1dby pressing that tube.

(6) Extra Length Control Step

When the metal tube covered optical fiber cable 12 is wounded around thecapstans 11a and 11b and is subjected to traction, a tension acts due toa frictional force between the metal tube 1d of the metal tube coveredoptical fiber cable 12 and the capstans 11a and 11b. This frictionalforce is large at the initial period of winding and is graduallyreduced. The tension is also large at the initial period of winding andis gradually reduced, accordingly. Elongation occurs in a wound portionof the metal tube 1d in correspondence with the tension.

Assume that, in a normal operation, the stainless steel strip 1 having awidth of 4 mm and a thickness of 0.1 (mm) is used, that the strip 1 isformed into a metal tube 1c having an outer diameter of 1.3 (mm), andthat the metal tube 1c is drawn into the metal tube 1d having an outerdiameter of 1.0 (mm). In this case, when a tension of the metal strip 1is adjusted by the tension adjusting means 14 such that a tension of themetal tube 1c at the inlet of the capstan 11a is set to about 20 (kgf),the tension causes elongation of the metal tube 1d by +0.30%. At thistime, when the tension of the optical fiber cable 5 having an outerdiameter of 125 (μm) is adjusted by the tension adjusting means 15 and atension of about 25 (gf) acts on the inlet side of the capstan 11a,elongation occurs by +0.03%.

Degrees of elongation of the metal tube 1d and the optical fiber cable 5are measured as a function of the number of turns of the metal tube 1dwound around the capstans 11a and 11b. The measurement results are shownin FIG. 14. The number of turns of the tube wound around the capstans11a and 11b is plotted along the abscissa, and an elongation (%) of themetal tube 1d is plotted along the ordinate. Referring to FIG. 14, acurve E represents characteristics of changes in elongation of the metaltube 1d, and a curve F represents characteristics of changes inelongation of the optical fiber cable 5. As indicated by the curve E,when the metal tube 1d is wound around the capstans 11a and 11b sixtimes, the final elongation of the metal tube 1d supplied to the tensionadjusting means becomes very small. As indicated by the curve F, whenthe optical fiber cable 5 is wound one and half times, elongation isalmost zero.

When the elongation of the optical fiber cable 5 becomes almost zero byits 1.5-time winding, an elongation of +0.19 is present in the metaltube 1d. Immediately after the metal tube 1d is wound around thecapstans 11a and 11b six times, the tension of the metal tube 1d becomesalmost zero. In this case, the elongation of the metal tube 1d becomesalmost zero accordingly. That is, when the tube wound around thecapstans six times, the metal tube 1d shrinks by 0.19% as compared with1.5-time winding. On the other hand, since the tension of the opticalfiber cable is almost zero upon 1.5-time winding, no change inelongation occurs, and the length of the optical fiber cable is keptunchanged. For this reason, 6-time winding causes an elongation of 0.19%in the optical fiber cable 5 as compared with the metal tube 1d.

The winding diameter of the metal tube 1d wound around the capstans 11aand 11b is different from that of the optical fiber cable 5 engaged withthe inner wall of the metal tube 1d. For this reason, when the diameterof each of the capstans 11a and 11b is about 500 mm, the optical fibercable 5 has an elongation amount corresponding to +0.09% with respect tothe metal tube 1d. This elongation amount of 0.09% is canceled with theabove 0.19%. As a result, the optical fiber cable 5 is longer than themetal tube 1d by 0.10%.

Assume that the tension of the metal tube 1d at the inlet side of thecapstan 11a is the same as that shown in FIG. 14 and that the tension ofthe optical fiber cable 5 is changed to increase the tension at theinlet side of the capstan 11a. In this case, a change in elongation ofthe optical fiber cable 5 is indicated by a curve F1 in FIG. 15. Whenthe optical fiber cable 5 is wound around the capstans 11a and 11b 3.5times, the tension is almost zero. On the other hand, the elongation ofthe metal tube 1d is 0.09% in 3.5-time winding. When the elongation of0.09% of the metal tube 1d is canceled with the elongation of 0.09% ofthe optical fiber cable 5, a difference between the lengths of thesemembers, i.e., an extra length, becomes 0%.

Contrary to the case in FIG. 15, when a tension of the metal tube 1d atthe inlet side of the capstan 11a is increased by applying a tension ofthe metal strip 1 by the tension adjusting means while the tension ofthe optical fiber cable 5 at the inlet of the capstan 11a is keptunchanged, a change in elongation in metal tube 1d is represented by acurve E1 in FIG. 16.

Assume that the tension of the metal tube 1d at the inlet side of thecapstan 11a is set equal to that in FIG. 14, and that a tension of themetal tube 1d at the outlet sides of the capstans 11a and 11b areincreased by the tension adjusting means 14. In this case, a change inelongation of the metal tube 1d is represented by a curve E2 in FIG. 16.A curve E3 in FIG. 16 represents a case wherein the tensions of themetal tube 1d at the inlet and outlet sides of the capstans 11a and 11bare increased.

As described above, one or both of the tensions of the metal tube 1d atthe inlet and outlet sides of the capstans 11a and 11b are increased, orboth the tensions are increased by a predetermined value. In this case,the length of the optical fiber cable 5 can be larger than that of themetal tube 1d by a desired amount. For example, as indicated by thecurve E3, when the metal tube 1d is wound around the capstans 11a and11b by one and half times, the elongation of the metal tube 1d becomes+0.26%. Even if the elongation of 0.09% of the optical fiber cable 5,which is caused by the winding diameter, is subtracted from theelongation of the metal tube 1d, the optical fiber cable 5 is longerthan the metal tube 1d by 0.17% at the outlet side of the capstan.

When a tension of the optical fiber cable 5 at the inlet side of thecapstan is set larger than that in FIG. 15, and a change in elongationof the optical fiber cable 5 is represented by a curve F2 in FIG. 17,the length of the optical fiber cable 5 can be set smaller than that ofthe metal tube 1d. In this case, the elongation of the optical fibercable 5 becomes almost zero in 5-time winding, and the correspondingelongation of the metal tube 1d becomes +0.04%. This elongation of+0.04% is subtracted from the winding difference of 0.09% in the opticalfiber cable 5. Therefore, the optical fiber cable 5 can be set shorterthan the metal tube 1d by 0.05%.

As described above, by systematically adjusting the capstans 11a and 11bwound with the metal tube covered optical fiber cable 12 by a pluralityof times, the tension adjusting means 14 for the metal strip 1, and thetension adjusting means 15 for the optical fiber cable 5, andoccasionally the tension adjusting means 13 in the downstream of thecapstans 11a and 11b, the length of the optical fiber cable 5 relativeto the metal tube 1d can be arbitrarily adjusted. When the tensions ofthe metal tubes 1c and 1d are adjusted by adjusting the positioning unit71 as in the tension at a predetermined extra length ratio can beperformed.

In the above embodiment, argon gas is used as an inert gas. However,nitrogen gas may be used to obtain the sam effect as described above.

In the above embodiment, a gel is not supplied to a metal tube forcovering the optical fiber cable. When the gel is supplied to the metaltube, a gel is supplied from the insert gas supply tube 63 in theoptical fiber cable guide means 6. In this manner, the gel can besupplied to the metal tube 1d by utilizing only one guide tube 61.

In this case, the inert gas and gel are supplied at a pressure whichdoes not apply a tension on the optical fiber cable 5 by the inert gasor gel flow because insertion of the optical fiber cable 5 and extralength control can be achieved, as they are desired, without supplyingthe inert gas and gel through the optical fiber cable guide means 6.

In the above embodiment, the optical fiber cable guide means 6 isarranged between the first and second assemblies 3a and 4 of theassembly 2. However, as shown in FIG. 18, the optical fiber cable guidemean 6 may be arranged in the downstream of the first assembly 3, andthe guide tube 61 may be inserted at the inlet of the first formingroller pair 31a.

In the above embodiment, the traction means comprising the capstans 11aand 11b of the direct adjusting means 14 for the metal strip 1, extralength control can be more precisely performed. In this case, the extralength control function of the positioning unit 71 is the same as thatof the tension adjusting means 14 of the metal strip, and a detaileddescription thereof will not be made.

In the above case, extra length control is performed when the metal tube1d has an outer diameter of 1.08 (mm) and a thickness of 0.1 (mm) andthe optical fiber cable 5 has an outer diameter of 0.125 (μm).Elongations (%) of the metal tube 1d at the inlet of the capstan 11 andelongations (%) of the optical fiber cable, which are obtained for a 0%extra length upon variable changes in outer diameter and thickness ofthe metal tube 1d and outer diameter of the optical fiber cable 5 aresummarized in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Metal Tube                          Difference                                Outer   Fiber                       in                                        Diameter/                                                                             Diameter  Metal Tube                                                                              Fiber   Peripheral                                Thickness                                                                             (mm)      Elongation                                                                              Elongation                                                                            Length (%)                                ______________________________________                                        2.4/0.2 1.6       0.24      0.10    0.07                                       1.7/0.15                                                                             0.25      0.26      0.07    0.15                                      0.7/0.1 0.25      0.35      0.20    0.05                                      ______________________________________                                    

As shown in Table 1, even if the metal tube 1d having an arbitrary sizeand the optical fiber cable having an arbitrary size are used, extralength control tension variable means 11 and the tension adjusting means13 is arranged in the downstream of the drawing means 9, and thetensions of the optical fiber cable 5 at the inlet and outlet sides ofthe capstans 11a and 11b and the tension of the optical fiber cable atthe inlet side of the capstans are adjusted by the capstans 11a and 11band the tension adjusting means 14, 15, and 13 while traction of themetal tube covered optical fiber cable 12 is kept performed, therebyperforming extra length control. However, as shown in FIG. 19, a pullingmeans 19 for pulling the metal tube 1d may be arranged at the inletsides of the capstans 11a and 11b in the traction means to arbitrarilyadjust the tension of the metal tube 1d at the inlet side of thecapstans.

For example, an endless capstan may be used as the pulling means 19, andthe metal tube 1d is pulled while being clamped between the endlesscapstans, so that the metal tube 1d can be pulled with a tensionrequired in a forming schedule. By adjusting a feed speed of the endlesscapstan, the tension of the metal tube 1d supplied to the capstan 11acan be arbitrarily controlled.

For example, when the length of the optical fiber cable 5 is set smallerthan that of the metal tube 1d, in the case of FIG. 17, since a tensionof the metal tube 1d at the inlet side of the capstan 11a cannot bereduced due to a forming schedule, a tension of the optical fiber cable5 at the inlet side is increased. However, an excessive increase intension of the optical fiber cable is not preferable. A tension of themetal tube 1d at the inlet side is decreased to obtain an effect whereina tension of the optical fiber cable 5 is relatively increased.Therefore, the length of the optical fiber cable 5 can be reducedwithout applying an excessive force to the optical fiber cable 5.

After the metal tube covered optical fiber cable 5 is manufactured andis to be fabricated in the subsequent process, an actual extra lengthmay become different from a target extra length. In this case, extralength control is also required. When the extra length control isperformed in advance in consideration of a deviation in extra lengthvalue, a metal tube covered optical fiber cable having an optimal extralength after the subsequent fabrication can be obtained.

In each embodiment described above, one optical fiber cable is insertedinto a metal tube. However, an optical fiber bundle consisting of aplurality of optical fibers can also be guided into a metal tube in thesame manner as described above.

As has been described above, according to the present invention, whenthe metal tube cover optical fiber cable is to be manufactured, thetension of the metal tube covered optical fiber cable at the inlet ofthe tension variable means is adjusted by the tensions of the sealedmetal tube and the optical fiber cable guided into the metal tube toobtain a difference in tension between the sealed metal tube and theoptical fiber cable guided in the metal tube. This difference in tensionis reduced by the tension variable means to provide a difference inelongation amounts between the sealed metal tube at the inlet of thetension variable means and the optical fiber cable guided in the metaltube. The length of the optical fiber cable relative to the metal tubecan be arbitrarily adjusted by this difference in elongation amount inaccordance with a given application condition. Therefore, the metal tubecovered optical fiber cable can be stably installed and used.

Since the capstan around which the metal tub covered optical fiber cableis wound a plurality of times is used as the tension variable means, thetensions of the sealed metal tube and the optical fiber cable in themetal tube can be arbitrarily reduced, thereby controlling the extralength of the metal tube covered optical fiber cable with accuracy.

Since the tension of the metal tube at the inlet of the tension variablemeans can be arbitrarily set variable by the tension means, extra lengthcontrol can be stably performed, and the extra length control range canbe increased.

We claim:
 1. An apparatus for manufacturing a metal tube covered optical fiber cable, comprising:an assembly, having a plurality of roller pairs, for causing both side edges of a metal strip to abut against each other to form the metal strip into a metal tube; laser welding means for radiating a laser beam to abutment portions of the metal tube to bond the abutment portions to obtain a sealed metal tube; optical fiber guiding means for guiding an optical fiber or an optical fiber bundle into the formed metal tube; traction means for continuously drawing the metal strip, the formed metal tube, and the sealed metal tube incorporating the optical fiber or optical fiber bundle through said assembly, said optical fiber guide means, and said laser welding means; and extra length control means comprising:first tension adjusting means, arranged upstream of said assembly, for variably chaning a tension of the metal strip on an assembly side and for adjusting a tension of the metal tube; second tension adjusting means, arranged upstream of an optical fiber guide port of said optical fiber guiding means, for variably adjusting a tension of the optical fiber cable; and traction means including tension variable means for reducing a tension of the metal tube covered optical fiber cable and for supplying the metal tube covered optical fiber cable.
 2. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 1, wherein said traction means includes means for continuously drawing the metal strip, the formed metal tube, and the sealed metal tube incorporating the optical fiber or optical fiber bundle through said assembly, said optical fiber guide means, said laser welding means, and drawing means.
 3. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 1 or 2, wherein said tension variable means comprises a capstan around which the metal tube covered optical fiber cable is wound a plurality of times.
 4. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 1, 2, or 3, wherein the tension of the metal tube covered optical fiber cable at an outlet side of said tension variable means is adjusted by said tension adjusting means arranged downstream of said tension variable means.
 5. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 1, 2, 3, or 4, wherein the tension of the metal tube covered optical fiber cable at the inlet side of said tension variable means is adjusted by tension means arranged upstream of said tension variable means.
 6. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 1, 2, 3, 4, or 5, wherein the tension of the optical fiber cable is adjusted to a predetermined value and the tension of the metal strip is set variable, thereby controlling the extra length.
 7. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 1, 2, 3, 4, or 5, wherein the tension of the metal strip is adjusted to a predetermined value and the tension of the optical fiber cable is set variable, thereby controlling the extra length.
 8. A method of manufacturing a metal tube covered optical fiber cable, comprising the forming step of forming a metal strip subjected to traction into a metal tube through a forming roller, the laser welding step of welding abutment portions of the formed metal tube with a laser beam and forming the formed metal tube into a sealed metal tube, and the optical fiber guide step of guiding an optical fiber or optical fiber bundle in the sealed metal tube, comprising:setting a tension of the metal strip before the assembly step variable to adjust a tension of the metal tube after a drawing step; setting a tension of the optical fiber cable before the optical fiber guide step variable and adjusting the tension of the optical fiber cable guided into the metal tube; reducing a tension of the metal tube covered optical fiber cable subjected to traction; and controlling an extra length of the metal tube cover optical fiber cable.
 9. A method of manufacturing a metal tube covered optical fiber cable according to claim 8, wherein the tension of the metal tube covered optical fiber cable is reduced by a capstan around which the metal tube covered optical fiber cable is wound a plurality of times.
 10. A method of manufacturing a metal tube covered optical fiber cable according to claim 8 or 9, wherein the step of reducing the tension of the metal tub covered optical fiber cable is performed after a tension is applied to the metal tube covered optical fiber cable.
 11. A method of manufacturing a metal tube covered optical fiber cable according to claim 8, 9, or 10, wherein the tension of the optical cable is adjusted to a predetermined value and the tension of the metal strip is set variable to control the extra length.
 12. A method of manufacturing a metal tube covered optical fiber cable according to claim 8, 9, 10, or 11, wherein the tension of the metal strip is adjusted to a predetermined value and the tension of the optical fiber cable is set variable, thereby controlling the extra length.
 13. An apparatus for manufacturing a metal tube covered optical fiber cable according to claim 2, wherein said tension variable means comprises a capstan around which the metal tube covered optical fiber cable is wound a plurality of times.
 14. A method of manufacturing a metal tube covered optical fiber cable according to claim 9, wherein the step of reducing the tension of the metal tube covered optical fiber cable is performed after a tension is applied to the metal tube covered optical fiber cable. 